Fill a glass with ice (a red wine glass works perfectly for this cocktail).
Add the limoncello.
Add in the torn basil leaves.
Top off with tonic water.
Give a little stir to combine.
Serve.
Notes
Even if you don't usually like tonic water, this drink might change your mind. My MIL wasn't a fan of tonic until this cocktail. The sweetness of the limoncello helps take away the bitterness that tonic sometimes has.
Recipe by My Man's Belly at https://mymansbelly.com/2015/08/07/limoncello-cocktails-at-villa-massa/